1. Field
This disclosure relates generally to transmission of digitally modulated wireless signals, and more specifically to transmission of wireless signals modulated with both spread spectrum and non-spread spectrum modulation.
2. Related Art
Direct sequence spread spectrum communications systems operate by encoding each data bit over a number of modulated channel symbols that are referred to as chips. Each channel symbol also encodes a pseudorandom data sequence. Although a direct sequence spread spectrum communications system includes modulation and receiving equipment that process the signal at the modulated chip rate, data throughput is reduced by the spread spectrum coding. Direct sequence spread spectrum receivers correlate a received signal with a reproduction of the pseudorandom data sequence to extract the encoded data. The operation of direct sequence, spread spectrum systems allows the communication of data with enhanced interference rejection.
One direct sequence spread spectrum communications application is the IEEE 802.15.4 communications standard. The incorporation of direct sequence spread spectrum signals in the IEEE 802.15.4 communications standard allows reliable communications over an extended range. This use of direct sequence spread spectrum signals, however, reduces the data throughput of IEEE 802.15.4 systems relative to the total bandwidth consumed by the system and the corresponding capacity of the processing hardware of the communications equipment.
Therefore, a technique that would allow the hardware used to implement an IEEE 802.15.4 transmitter to communicate at a higher data rate throughput than is supported by that standard expands the usability of that hardware.
The IEEE 802.15.4 protocol specifies that the 2.4 GHz physical layer be implemented with offset quadrature phase shift keying (O-QPSK) modulated with half-sine pulse shaping. This modulation format is also known as minimum shift keying (MSK). Even-indexed chips are modulated onto an in-phase (I) carrier and odd-indexed chips are modulated onto a quadrature-phase (Q) carrier. Because each data symbol is represented by a 32-chip sequence, the chip rate is 32-times the symbol rate. The IEEE 802.15.4 protocol specifies a bit rate of 250 kbps and a chip rate of 2000 kcps, which means that protocol has a spreading gain of eight (8).
FIG. 1 illustrates a prior art packet 100 used in the protocol of the IEEE 802.15.4 standard. The prior art packet 100 comprises a preamble 101, a start of frame delimiter (SFD) 102, a payload data length (PHR) portion 103 and a payload portion 104. The protocol of the IEEE 802.15.4 standard specifies that the data of the entire prior art packet 100 is transmitted using spread spectrum modulation. The preamble 101 of the prior art packet 100 comprises eight (8) symbols. The payload portion 104 comprises up to 256 symbols. Each symbol comprises four (4) bits. Each bit comprises eight (8) chips.